Water–energy–carbon synergies and trade-offs: A daily nexus analysis for wastewater treatment plants

Given the increasing interest in achieving carbon neutrality, water–energy–carbon synergy and trade-off mechanisms are critical for the sustainability of wastewater treatment plants (WWTPs). Based on daily operation data of a WWTP in southern China from 2007 to 2021, we propose a new approach to explore the water–energy–carbon (WEC) nexus of WWTPs by developing a Water–Energy-Carbon Coupling Index (WECCI) and a four-quadrant clustering (FQC) method. It is found that the WECCI increased by 10.12% after the first standard upgrade and decreased by 8.20% after the second standard upgrade, which indicates that the transformation of WWTP may lead to unexpected change in the overall performance. Based on the FQC method, 85.64% of the operation data are mainly concentrated in the WEC best synergy zone after the first standard upgrade, whereas 58.52% of the operation data are concentrated in the low carbon trade-off zone after the second standard upgrade. This study indicates that WECCI and FQC are necessary tools to quantitively assess the integrated performance of WWTPs for better understanding the complex WEC mechanisms of WWTPs, which facilitate the sustainable transition of WWTPs towards carbon neutrality.